That is a generic error message, which could be caused by at least a few different things we're aware of, which we have listed on our Help page here:
https://www.e-junkie.com/wiki/trouble-paypal-checkout-error
That said, we made a test purchase for one of your products using the guest checkout with card and were able to complete the purchase without issue. So perhaps the problem has already resolved itself.
If the issue continues you may want to contact PayPal support to see if there is any issue on their end.
A couple last things we'll note here; when testing your own purchase buttons, note that PayPal won't let you "pay yourself", so you can't test checkout with the same PayPal account you'd be using to receive the payment, nor with a card issued in your own name.
Additionally, PayPal "guest checkout" for card payment without a PayPal account also won't work with any card that's already linked to a PayPal account. So the buyer would need to log into their PayPal account that already has their preferred payment card linked to it, or select a different card to complete checkout as guest.
